The present disclosure relates generally to frequency synthesizers. Frequency synthesizers generate frequencies from one or more fixed reference frequencies, and are found in various devices including musical instruments, GPS systems, mobile telephones, etc.
There are several different types of frequency synthesizers including direct analog synthesizers, direct digital synthesizers, and indirect digital synthesizers. The indirect digital synthesizers based on phase-locked loops (“PLLs”) are compatible with integrated circuit technology. The indirect digital PLL synthesizers often include the following components: voltage controlled oscillators, mixers, PLLs, frequency multipliers, and frequency dividers. A voltage-controlled oscillator of a PLL synthesizer typically generates an output frequency from the filtered output of the phase frequency detector. A divider then scales the output frequency. In some applications, a reference frequency is dynamic and can span a multi-decade range of frequency values. In these cases, the traditional PLL synthesizers have drawbacks.